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Gearbox Oil

Started by amgba, August 09, 2022, 01:17:12 PM

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I am getting conflicting advice on the B gearbox. How much and what kind of oil I should use? Book says 20W50 but a tech bulletin says 75w90 or 80W90.

Overdrive trans people say 30w non detergent oil.

John Brower
American MGB Association
phone/text: 773-769-7084

North America's oldest and largest club for all MGBs, MGB-GTs & Midgets.  A nonprofit organization founded in 1975 and offers color magazines, emagazines, technical advice, member recommendations on service shops, registration of your MG, a tradition of service & FREE member classified ads that appear in the magazine & on the website.


This question comes up all the time.  First of all, any oil is better than no oil.  During my time in the shop we drained out ATF, engine oil, and differential/hypoid gear lube from the gearboxes.  The factory workshop manual 1955 MGA through 1980 MGB (and MGC) instructs the use of engine oil – 20W/50.  For some reason, about 1978, there was a service memorandum circulated that said "Oh my gosh, these gearboxes should be using 80/90."  Then, six months later, another memorandum came out saying, "Oh my gosh, we don't know why we wrote that, use engine oil."  That incorrect memorandum still circulates.

Now, John Esposito of Quantum Mechanix, who rebuilds jillions of BMC gearboxes, instructs his customers to use 30 weigh non-detergent oil, especially in overdrives.  I used engine oil 20W/50 for fifty years in my customers' gearboxes.  Use the high zinc oil (e.g. Valvoline VR-1) in the engine – there's no need to use high zinc in the gearbox.
So – it's either engine oil or 30 SAE.  Your call.

Hope this helps.
John H Twist
John H Twist
University Motors Online
100 East Beltline Avenue SE
Grand Rapids, MI  49506


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